The present invention relates to the field of powering ports, and more particularly to a powering circuit having a bypass path to prevent excess voltage during port disconnect.
The growth of local and wide area networks based on Ethernet technology has been an important driver for cabling offices and homes with structured cabling systems having multiple twisted wire pairs. The ubiquitous local area network, and the equipment which operates thereon, has led to a situation where there is often a need to attach a network operated device for which power is to be advantageously supplied by the network over the network wiring. Supplying power over the network wiring has many advantages including, but not limited to; reduced cost of installation; centralized power and power back-up; and centralized security and management.
Several patents addressed to this issue exist including: U.S. Pat. No. 6,473,608 issued to Lehr et al., whose contents are incorporated herein by reference and U.S. Pat. No. 6,643,566 issued to Lehr et al., whose contents are incorporated herein by reference. Furthermore a standard addressed to the issue of powering remote devices over an Ethernet based network has been published as IEEE 802.3af-2003, whose contents are incorporated herein by reference.
An Ethernet switch or midspan module providing power over Ethernet functionality is typically designed to support a plurality of ports, and power is preferably to be supplied to compatible equipment after detection. Power is typically supplied under control of a power over Ethernet controller, the power over Ethernet controller energizing for each port to be powered an electronically controlled switch, which in an exemplary embodiment comprises a power MOSFET. In another embodiment the electronically controlled switch comprises a FET or bipolar transistor. In order to reduce cost and minimize the footprint, preferably the required electronically controlled switches are provided embedded within the power over Ethernet controller. In another embodiment the required electronically controlled switches are provided external to the power over Ethernet controller, and are responsive to an output of the power over Ethernet controller.
After powering a port for which a valid attached powered device has been detected, the port is monitored for a valid maintain power signature (MPS). The above mentioned standard describes two MPS components; an AC MPS component and a DC MPS component. The power over Ethernet controller may optionally monitor the AC MPS component, the DC MPS component or both the AC and the DC MPS components. Implementation of the AC MPS component requires an AC signal source to be connected to the port. In the event that the power over Ethernet controller detects an absence of a valid monitored MPS component, power to the port is to be disconnected. Preferably, disconnection is to occur within 300-400 ms of the dropout of a valid monitored MPS component.
FIG. 1 illustrates a high level schematic diagram of a power over Ethernet controller arranged to monitor an AC MPS component for disconnection of a powered device according to the prior art. The system of FIG. 1 comprises power over Ethernet controller 10, powered device 20, first twisted pair 30, second twisted pair 35, power source PS1, sense resistor Rsense, unidirectional current means D1, output impendence Zout and output capacitor Cout. Power over Ethernet controller 10 comprises control circuit 40, AC signal source 50, AC signal source resistance Rac, electronically controlled switch SW1, detection source Idetect, control means 60, control means 70, sensing input 80 and control means 90. Powered device 20 comprises Cload and Zload. Electronically controlled switch SW1 is illustrated as a power MOSFET, however this is not meant to be limiting in any way. SW1 may be implemented as a FET or bipolar transistor without exceeding the scope of the invention. Detection source Idetect is illustrated as being a variable current source, however this is not meant to be limiting in any way. Detection source Idetect may be implemented as a voltage source or as a plurality of current sources without exceeding the scope of the invention. Twisted pairs 30 and 35 form part of a single structured communication cabling. Cload and Zload schematically represent the input capacitance and load, respectively, of powered device 20. In an exemplary embodiment Zout comprises a 45.3 K resistor and Cout comprises a 0.2 μf capacitor.
Switch SW1 is illustrated as being internal to power over Ethernet controller 10, typically as part of a single integrated circuit, however this is not meant to be limiting in any way. Switch SW1 may be implemented externally to power over Ethernet controller 10 without exceeding the scope of the invention. Control means 60 may be a direct output of control circuit 40 or a circuit responsive thereto without exceeding the scope of the invention.
The positive of PS1 is connected to the anode of unidirectional current means D1 and a first end of Zout. The cathode of unidirectional current means D1 is connected to the positive side of Cout, control circuit 40 via sensing input 80, one end of Rac and a first end of first twisted pair 30. A second end of Rac is connected to the output of AC signal source 50, and the return of AC signal source 50 is connected to ground. The control input of AC signal source 50 is connected to control circuit 40 via control means 90. The control input of detection source Idetect is connected to an output of control circuit 40 via control means 70. The gate of electronically controlled switch SW1 is connected to an output of control circuit 40 via control means 60. The negative of PS1 is connected to ground and one end of Rsense. A second end of Rsense is connected to an input of control circuit 40 and to the drain of SW1. The source of SW1 is connected to one end of detection source Idetect, to the second end of Zout, the negative side of Cout and a first end of second twisted pair 35. The return of detection source Idetect is connected to ground. A second end of first twisted pair 30 is connected to one end of Zload and the positive side Cload. A second end of Zload and the negative side of Cload are connected to a second end of second twister pair 35.
In operation control circuit 40 operates detection source Idetect through control means 70 to generate a plurality of current levels. The plurality of current levels flow through Zload, if connected, thereby presenting a plurality of voltages sensed at sensing input 80. After detection and classification of a valid powered device 20, power from PS1 is connected over first and second twisted pairs 30,35 by the operation of control means 60 to electronically controlled switch SW1. AC signal source 50, operated via control means 90, supplies an AC MPS which is sensed at sensing input 80. Among other functions, unidirectional current means D1 prevents the attenuation of the output of AC signal source 50 by blocking a connection to PS1. Upon detection of an invalid MPS, i.e. the absence of a valid MPS, control circuit 40 operates control means 60 to open electronically controlled switch SW1 thereby disabling power to the port.
It is to be understood by those skilled in the art that just prior to opening electronically controlled switch SW1 capacitor Cout is charged to nearly the output voltage of PS1. In an exemplary embodiment, the output voltage of PS1 is 48 volts. Upon the opening of electronically controlled switch SW1 no discharge path is available for Cout through Zout, and thus the negative side of Cout is at the same potential as the positive output of PS1 in relation to ground. In particular, the positive side of Cout is at approximately twice the potential of PS1 in relation to ground. Since the positive side of Cout is connected to sense input 80, control circuit 40 experiences an excess voltage well in excess of its normal operating range. Such an excess voltage can lead to damage or require the use of a substantial transient surge suppressor.
What is needed, and not supplied by the prior art, is a means from preventing excess voltage from appearing at an input to a power over Ethernet controller monitoring an MPS component by providing a path for discharging the output capacitor connected to the power over Ethernet controller.